Diffusion mechanism of cesium ions in compacted montmorillonite

摘要

For the safety assessment of geological disposalof high-level radioactive waste, the diffusion behaviour of Cs+ions in compacted Na-and Ca-montmorillonite mixtures wasstudied. The apparent diffusion coefficients of Cs+ ions weredetermined as functions of the diffusion temperature and theionic equivalent fraction of Cs2+ ions. The apparent diffusioncoefficients were almost constant at higher temperatures, 323 and313K, while they decreased with increasing ionic equivalentfraction at lower temperatures, 298 and 288 K. The activationenergies for diffusion of Cs+ ions were also determined from thetemperature dependence of the apparent diffusion coefficients ateach ionic equivalent fraction. The activation energy increasedfrom 34 kJ mol-1 to 38kJ mol-1 as the ionic equivalent fractionincreased from zero to 0.75, after which it increased steeply andreached 49 kJ mol-1 at the ionic equivalent fractions of 0.94 and1.0. These findings cannot be explained by the pore water orsurface diffusion models. Interlayer diffusion is suggested as themost probable explanation for the diffusion of Cs+ ions.